The western escarpment of the Sea of Marmara has recently been recognized as the site of intensive gas emissions escaping from the seafloor. Visual observations with the Nautile submersible indicate that gas escapes from elongated tensile cracks oriented to the northwest in the direction of the maximum principal stress. Here, we report results from a 25-day test in 2007 with four ocean-bottom seismometers (OBSs) showing that this area is also characterized by microseismic activity. A cluster of 13 small-magnitude earthquakes aligned northwest occurred in less than 30 hr at shallow crustal depth below the western slope of the Tekirdag basin. The only two focal mechanisms resolvable using land and sea-bottom data reveal normal faulting with strike-slip components, consistent with the stress field expected in this area. It is suggested that tectonic strain below the western slope of the Tekirdag basin contributes to maintaining a high permeability in fault zones and that the fault network provides conduits for deep-seated fluids to rise up to the seafloor.